High-Level Design (HLD): What It Covers and a Practical Example in C#

 

When embarking on a software project, a well-thought-out High-Level Design (HLD) is crucial to guide development and ensure a scalable and maintainable system. It’s the bridge between abstract requirements and detailed implementation. But what exactly does HLD include, and how do you create one?

In this blog, we’ll break down the components of HLD, provide a real-world example, and explore how to represent them in C#.

What is High-Level Design (HLD)?

HLD provides a blueprint of the system architecture. It defines the structure, components, and interactions at a macro level, focusing on "what" the system should do rather than "how" it does it.

What HLD Includes

1. System Architecture:

   • The overall structure of the system, including its major components and their relationships.
   • Diagrams like component diagrams or architecture flow diagrams are often used.

2. Modules and Subsystems:

   • A breakdown of the system into modules or subsystems, each responsible for specific functionality.

3. Data Flow:

   • How data moves between components, including API contracts and data storage mechanisms.

4. Technology Stack:

   • The technologies, frameworks, and tools to be used for the implementation.

5. Integration Points:

   • Interfaces and protocols for communication between components and external systems.

6. High-Level Use Cases:

   • Major workflows or user interactions with the system.

7. Non-Functional Requirements (NFRs):

   • Performance, scalability, reliability, and security considerations.

Example Scenario: Online Bookstore System

Let’s design an Online Bookstore that allows users to browse books, add them to a cart, and make purchases.

High-Level Design for Online Bookstore

1. System Architecture

   • The system consists of the following components:
     • Web Application: User-facing frontend.
     • API Layer: Backend services for business logic.
     • Database: Stores book, user, and order data.
     • Payment Gateway: Third-party integration for processing payments.

2. Modules

   • User Management: Handles user accounts, login, and registration.
   • Book Catalog: Manages book data and search functionality.
   • Cart and Checkout: Handles cart operations and order placement.

3. Data Flow

   • User requests flow from the frontend to the API layer, which interacts with the database and external systems like the payment gateway.

4. Technology Stack

   • Frontend: Angular or React.
   • Backend: .NET Core (C#).
   • Database: SQL Server or MongoDB.
   • Payment Gateway: Stripe or PayPal API.

5. Integration Points

   • APIs for frontend-backend communication.
   • Webhooks for payment gateway interactions.

6. Use Case: Place an Order

   • User selects books and adds them to the cart.
   • User proceeds to checkout and makes a payment.
   • Backend processes the payment, confirms the order, and updates the database.

C# Implementation Example

Here’s how some of these components might look in code.

1. Models

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public decimal Price { get; set; }
}

public class User
{
    public int UserId { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
}

public class Order
{
    public int OrderId { get; set; }
    public int UserId { get; set; }
    public List<Book> Books { get; set; }
    public decimal TotalAmount { get; set; }
    public DateTime OrderDate { get; set; }
}

2. Service Layer

Book Catalog Service

public class BookCatalogService
{
    private readonly List<Book> _books = new List<Book>
    {
        new Book { BookId = 1, Title = "C# in Depth", Author = "Jon Skeet", Price = 49.99m },
        new Book { BookId = 2, Title = "Clean Code", Author = "Robert C. Martin", Price = 39.99m }
    };

    public List<Book> GetBooks() => _books;

    public Book GetBookById(int bookId) => _books.FirstOrDefault(b => b.BookId == bookId);
}

Order Service

public class OrderService
{
    public Order PlaceOrder(User user, List<Book> books)
    {
        if (books == null || !books.Any())
            throw new ArgumentException("Cart is empty.");

        var order = new Order
        {
            OrderId = new Random().Next(1000, 9999),
            UserId = user.UserId,
            Books = books,
            TotalAmount = books.Sum(b => b.Price),
            OrderDate = DateTime.Now
        };

        // Save to database logic goes here (omitted for simplicity)

        Console.WriteLine($"Order placed successfully for {user.Name}. Total: ${order.TotalAmount}");
        return order;
    }
}

3. Controller

public class BookstoreController
{
    private readonly BookCatalogService _bookCatalogService;
    private readonly OrderService _orderService;

    public BookstoreController(BookCatalogService bookCatalogService, OrderService orderService)
    {
        _bookCatalogService = bookCatalogService;
        _orderService = orderService;
    }

    public void BrowseBooks()
    {
        var books = _bookCatalogService.GetBooks();
        Console.WriteLine("Available Books:");
        books.ForEach(book => Console.WriteLine($"{book.BookId}. {book.Title} by {book.Author} - ${book.Price}"));
    }

    public void PlaceOrder(User user, List<int> bookIds)
    {
        var books = bookIds.Select(id => _bookCatalogService.GetBookById(id)).Where(book => book != null).ToList();
        _orderService.PlaceOrder(user, books);
    }
}

4. Client Code

public class Program
{
    public static void Main()
    {
        var bookCatalogService = new BookCatalogService();
        var orderService = new OrderService();
        var controller = new BookstoreController(bookCatalogService, orderService);

        controller.BrowseBooks();

        var user = new User { UserId = 1, Name = "Alice", Email = "alice@example.com" };
        controller.PlaceOrder(user, new List<int> { 1, 2 });
    }
}

What Makes a Good HLD?

1. Clarity: The design should be easy to understand, even for non-technical stakeholders.
2. Scalability: The architecture should handle future growth with minimal changes.
3. Flexibility: Components should be loosely coupled to support independent modifications.

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Conclusion

High-Level Design (HLD) serves as the foundation for building robust systems. It provides an overarching view of the architecture, breaking down the system into manageable components. By starting with a solid HLD, developers can ensure their system is scalable, maintainable, and aligned with user requirements.

The example above demonstrates how to design and implement an Online Bookstore with HLD concepts in C#. While the implementation may vary, the principles of HLD—clarity, modularity, and scalability—remain constant.

Got unique thoughts or improvements? Let us know in the comments! 🚀